1. Field of the Invention
This invention relates to a catalyst composition or system. More particularly, this invention relates to a catalyst system for the production of carbonylation products by the reaction of a carbonylatable reactant and carbon monoxide which comprises (1) a rhodium-containing catalytically active component and (2) a halogen-containing promoter component, the halogen being selected from the group consisting of bromine, iodine, bromide compounds, and iodide compounds.
2. Description of the Prior Art
Carbonylation catalysts for the production of carbonylation products are known in the art, with many being based on cobalt and using a halide promoter. Examples of such catalysts are described in U.S. Pat. No. 2,789,137 and U.S. Pat. No. 2,730,546, both to Reppe et al.
The Reppe et al '137 patent discloses the production of acetic anhydride from dimethyl ether or methyl acetate by reaction of either of these compounds with carbon monoxide in an anhydrous liquid medium having dissolved therein cobalt bromide, cobalt iodide, or a mixture thereof. Reaction conditions specified include a pressure which "should be in excess of 200 atmospheres (2.026.times.10.sup.4 kPa, 2940 psi), with pressures between 400 and 800 atmospheres (4.053.times.10.sup.4 -8.11.times.10.sup.4 kPa, 5880-11,760 psi) being preferred, and temperatures in the range of 100.degree.-250.degree. C., preferably between 150.degree. C. and 200.degree. C. The catalysts can also be in the form of cobalt metal itself or other cobalt compounds in combination with free bromine, or iodine or of compounds thereof such as alkyl bromides or iodides. Additional inert solvents such as the lower N-alkylpyrrolidones, in particular, N-methylpyrrolidone, acetic acid, or acetic anhydride itself are used to maintain the catalyst at least partly in the dissolved state.
The Reppe et al '546 patent is directed to the synthesis of low-molecular weight saturated aliphatic carboxylic acids, their esters and anhydrides by treating lower fatty acid esters of lower aliphatic saturated alcohols such as methyl acetate with carbon monoxide-containing gases under superatmospheric pressure at elevated temperatures in the presence of complex cobalt halides which contain in the molecule, in addition to the cobalt halide, an organic onium halide, for example, an ammonium or phosphonium halide. Reaction temperatures between 70.degree. C. and 250.degree. C. are preferred but higher temperatures, for example, up to 300.degree. C. may also be used. Pressures are superatmospheric, preferably above 50 atmospheres (5.066.times.10.sup.3 kPa, 735 psi) and most preferably in the range from 200 to 300 atmospheres (2.026.times.10.sup.4 kPa to 4.053.times.10.sup.4 kPa, 2940 psi to 5880 psi), although there reportedly is no principal obstacle against using higher pressures such as 700 or 800 atmospheres (7.093.times.10.sup.4 or 8.11.times.10.sup.4 kPa, 10,290 or 11,760 psi).
Certain disadvantages of the catalysts described in the prior art are catalyst instability, lack of product selectivity, and low levels of catalyst activities. One particular disadvantage of such catalysts results from their reliance or dependence upon metal carbonyls or modified metal carbonyls, such as dicobalt octacarbonyl, iron carbonyl, and nickel carbonyl, all of which require the use of high partial pressures of carbon monoxide to remain stable under the necessarily high reaction temperatures employed during carbonylation reactions. Of particular importance is dicobalt octacarbonyl which requires partial pressures of carbon monoxide as high as 2.068.times.10.sup.4 kPa-G to 6.839.times.10.sup.4 kPa-G (3,000 psig to 10,000 psig) under carbonylation conditions of 175.degree. C. to 300.degree. C.
Still another disadvantage of the carbonylation catalyst systems of the prior art is their relatively low level of activity. This low level of activity requires high catalyst concentrations, long reaction times, and high temperatures to obtain substantial reaction rates and conversions. Consequently, very large and costly processing equipment is required.
Another disadvantage of the catalyst systems of the prior art is their inability to maintain high selectivity to the desired carbonylation product at temperatures required for high conversion levels and high reaction rates. At these higher temperatures, undesirable by-products such as carbonylation products of higher carbon number than desired, carbon dioxide, methane, and water are formed, thereby resulting in substantial yield losses and necessitating additional product purification and recycle steps in the processing.
Accordingly, research efforst are continually being made to define new or improved catalyst systems and methods and processes of preparing highly active new catalyst systems having high selectivities to the desired products in particular processes. The discovery of the catalyst system of the instant invention which does provide a more reactive and more stable carbonylation catalyst system than those known to the prior art is therefore believed to be a decided advance in the catalyst art.